The instant description will extensively rely on acronyms/abbreviations that are common in the technical area considered. The following is a list of those acronyms/abbreviations provided by way of direct reference, along with short explanations of the corresponding meanings:
xDSL X-Digital Subscriber Line
POTS Plain Old Telephone Service
PTSN Public Switching Telecommunication Network
ISDN Integrated Service Digital Network
CO Central Office
CPE Customer Premise Equipment
DS Down Stream
UP Upstream
HV High Voltage
LV Low Voltage
LPF Low Pass Filter
HPF High Pass Filter
COMBO Integrated POTS and ADSL line cards
PSD Power Spectral Density
VCCS Voltage Controlled Current Source
ELF Extremely Low Frequencies
PAD Phase Adjuster filter
SMD Surface Mounted Device
GIC Generalized Immittance Converter
FIG. 1 shows the general layout of a “splitter” for xDSL systems that separates xDSL and POTS/ISDN systems by means of a bi-directional filtering action. The acronyms shown in FIG. 1 and the functions performed by the functional blocks designated thereby are well known to those of skill in the art, thus making it unnecessary to provide a detailed description herein
XDSL technology can co-exist with other communication channels like POTS and ISDN, which share the same cable/loop placed between CO and CPE as shown in FIG. 1
The typical bandwidth used in xDSL applications ranges from 20-130 KHz to 1-11 MHz whereas POTS/ISDN applications involve frequencies from DC to 20-130 KHz. Different signals are allocated in the low frequency band too (DC—50 Hz), such as:                line test signals,        ringer,        dial pulsing,        on/off hook signals, and        in specific cases, billing tones (12-16 KHz).        
The basic requirements for a splitter are well known in the art, as witnessed e.g. by the above-referenced ITU-T Recommendation and by additional documents (which are incorporated by reference) such as:
ETSI TR 101 728 (V1.1.1): “Access and Terminals (AT); Study for the specification of the low pass section of POTS/ADSL splitters”.
ANSI T1.413: “Network to Customer Installation Interfaces—Asymmetric Digital Subscriber Line (ADSL) Metallic Interface”.
ETSI TS 101 388 (V1.2.1): “Transmission and Multiplexing (TM); Access transmission systems on metallic access cables”.
ETSI TS 101 952 (V1.1.1): “Part 1: ADSL splitters for European deployment;
Sub-part 3: Specification of ADSL/ISDN splitters”.
ETSI TS 101 952 (V1.1.1): Part 1: ADSL splitters for European deployment; Sub-part 4: Specification of ADSL over “ISDN or POTS universal splitters.
ITU-T Brugge, Belgium, 17-21 Jun. 2002: “Draft Rec. G.992.3-ADSL2”.
ITU-T Recommendation G.992.5 ASYMMETRIC DIGITAL SUBSCRIBER LINE (ADSL) TRANSCEIVERS-EXTENDED BANDWIDTH ADSL2 (ADSL2plus).
COMMITTEE T1—TELECOMMUNICATIONS Working Group T1E1.4 (DSL Access): Very-high-bit-rate Digital Subscriber Line (VDSL) Metallic Interface Part 1: Functional Requirements and Common Specification.
ETSI TS 101 952-2-3 v1.1.1: “Part 2: VDSL splitters for European deployment; Sub-part 3: Specification of VDSL/ISDN splitters for use at the Local Exchange (LE) and the user side near the Network Termination Port (NTP)”.
ETSI TS 101 952-2-1 v1.1.1: “Part 2: VDSL splitters for European deployment; Sub-part 1: Specification of the low pass part of VDSL/POTS splitters”.
The basic aim of a splitter is to separate xDSL signals from POTS/ISDN signals. Specifically, the splitter attenuates spurious signals falling in the xDSL band due to POTS/ISDN events like on/off-hook, ringer, ring injection, ring trip dial pulses and so on, while vice versa avoids that xDSL signals may reach the POTS/ISDN port showing high impedance in the xDSL band.
A splitter is usually comprised by a HPF and a LPF: the former is used as xDSL-loop interface by cutting out frequencies lower than 30 KHz-130 KHz, the latter acts as a POTS/ISDN-loop interface avoiding noise injection in xDSL band (from POTS to loop) and providing, in the same bandwidth, a high impedance seen from loop to POTS.
Usually, ADSL CO modems already include a CO HPF. In the rest of the instant description, the term splitter applies primarily to the LPF part only.
Reference works such as, e.g. J. Cook and P. Sheppard: “ADSL and VDSL Splitter Design and Telephony Performance”, IEEE Journal on selected areas in communications, Vol. 13, No. 9, December 1995, XDSL, which is incorporated by reference, show that splitters can be divided into passive and active categories.
In the passive case, a relatively huge number of HV passive devices is often necessary to produce a filter of adequate order (4th-7th) see, e.g. U.S. Pat. No. 6,144,735 or U.S. Pat. No. 5,627,501, which are incorporated by reference.
In the active case—see, e.g. U.S. Pat. No. 5,623,543 or U.S. Pat. No. 6,628,783, which are incorporated by reference—active devices permit a reduction in the order of the passive filter.
Other arrangements—see, e.g. U.S. Pat. No. 5,627,501 (already cited in the foregoing) or U.S. Pat. No. 5,889,856, which is incorporated by reference, require digital signal processing.
It has been observed that both the article of Cook and Sheppard and U.S. Pat. No. 5,623,543 provide an active approach that implements high-performance splitters based on GIC and a line-voltage sensing with two HV capacitors. But these arrangements do not take into account complexity, density, and power consumption (e.g., 80 mW quiescent power).
U.S. Pat. No. 5,627,501 adopts a passive approach implemented with a low-pass filter (LPF) plus a variable impedance circuit. A measurement unit placed on the ADSL side detects the LPF coil saturation and inserts additional impedance that limits the POTS current flow. This solution is not applicable for a stand-alone splitter requiring the ADSL system supervision; additionally, expensive additional components such as switches must be added.
U.S. Pat. No. 6,144,735 provides a solution based on a configurable filter. It makes extensive use of passive components plus an off-hook detector network that switches the splitter in two different filtering states.
U.S. Pat. No. 6,628,783 provides an active solution without supply and able to suppress (with a depletion MOS device) high frequency spurious signals due to POTS events. It requires a HV MOS with low RON value and a 3rd-order extra passive section to satisfy standard splitter requirements.
U.S. Pat. No. 5,889,856 discloses a purely digital approach applicable only to COMBO arrangements that offer a single termination point on the CO-side. An analog splitter is not required, but the complexity of the digital part is increased. Additionally, analog circuitry (which is not easy to integrate) is required to provide DC feed, battery voltage, and ring.